I need help on Streamreader and Streamwriter, please

[asem0525]

I have this kind of exception:
System.IO.IOException: The process cannot access the file 'E:\asem\Visual Studio 2005\WebSites\WebSite1\App_Data\Books.txt' because it is being used by another process.

and don't know the solution.
Here is the code, somebody help me please, and thank you in advance.

protected void AddBook()
{
string strISBN = tbISBN.Text;
string filePath = Server.MapPath("Books.txt");

FileStream file = new FileStream(filePath, FileMode.Append, FileAccess.Write);

try
{
StreamReader sr = new StreamReader(filePath);
content = sr.ReadToEnd();
sr.Close();

if (content.Contains(strISBN))
{
lblUploadResult.Text = "isbn contained";
}
else
{
StreamWriter sw =new StreamWriter(filePath);
sw.WriteLine(tbTitle.Text + "," + tbISBN + "," + tbAuthor + "," + tbPublisher + "," + tbPrice + ",");
sw.Close();
}
}
catch (Exception) { }
}

[sani723]

file.Close();

[asem0525]

I'm sorry, that code wasn't the correct one, here is it:

StreamReader sr = File.OpenText(filePath);
        StreamWriter sw = File.AppendText(filePath);
 
        try
        {
            content = sr.ReadToEnd();
            if (content.Contains(strISBN))
            {
                lblUploadResult.Text = "isbn contained";
            }
            else
            {           
                sw.WriteLine(tbTitle.Text + "," + tbISBN + "," + tbAuthor + "," + tbPublisher + "," + tbPrice + ",");
 
            }
        }
        catch (Exception) { }
        sr.Close();

and it shows me the exception: System.IO.IOException: The process cannot access the file 'E:\asem\Visual Studio 2005\WebSites\WebSite1\App_Data\Books.txt' because it is being used by another process.